Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

From: Ingo Molnar <mingo@kernel.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rik van Riel <riel@redhat.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Mike Galbraith <umgwanakikbuti@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	X86 ML <x86@kernel.org>,
	williams@redhat.com, Andrew Lutomirski <luto@kernel.org>,
	fweisbec@redhat.com, Peter Zijlstra <peterz@infradead.org>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry
Date: Thu, 7 May 2015 14:29:13 +0200	[thread overview]
Message-ID: <20150507122913.GA17443@gmail.com> (raw)
In-Reply-To: <20150507121848.GB32271@lerouge>

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> > > We cannot take the lock_trace(task) from irq context, and we 
> > > probably do not need to anyway, since we do not care about a 
> > > precise stack trace for the task.
> > 
> > So one worry with this and similar approaches of statistically 
> > detecting user mode would be the fact that on the way out to 
> > user-space we don't really destroy the previous call trace - we 
> > just pop off the stack (non-destructively), restore RIPs and are 
> > gone.
> > 
> > We'll need that percpu flag I suspect.
> 
> Note we have the context tracking state which tells where the 
> current task is: user/system/guest.

Yes, but that overhead is what I'm suggesting we get rid of, I thought 
Rik was trying to find a mechanism that would be independent of that?

Thanks,

	Ingo